s-Triazines are a family of herbicides that has a symmetric heterocyclic aromatic ring with 6 alternating carbon and nitrogen atoms. Simazine and atrazine are the most relevant members of the s-triazine herbicide family among other less-used compounds such as hydroxysimazine, deethylhydroxysimazine, hydroxyatrazine, deethylatrazine, deethylhydroxyatrazine, deisopropylatrazine, fluoroatrazine, propazine, terbutylazine, cyanuric acid and cyanazine. Both simazine (IUPAC: 6-chloro-N2,N4-diethyl-1,3,5-triazine-2,4-diamine) and atrazine (IUPAC: 6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) are s-triazine herbicides that have a chlorine atom bound to the heterocyclic ring. The mechanism of this herbicide family comprises the inhibition of electron transport in the photosynthesis process.
s-Triazines, especially simazine and atrazine, have been widely used for the control of weeds in many agricultural regions of the world. In Chile, simazine is commonly used for agricultural and forestry practices.
In spite of their advantageous use as herbicides, s-triazines are environmental contaminants that can affect human health and ecosystems. It has been shown that atrazine and other s-triazines can induce breast cancer in rats. Therefore they have been classified as “possible human carcinogen” class-C by EPA (Tappe et al., 2002). Various ecosystems have been chemically perturbed by the constant use of s-triazine herbicides, especially simazine and atrazine. Due to their high mobility, these compounds can cause serious soil and ground water contamination. Therefore these herbicides frequently exceed the standard advisory levels of 0.1 μg L−1 and 3.0 μg L−1 by the European Union and USA regulatory authorities, respectively (Rousseaux et al., 2001).
Despite this background, the global manufacture and sales of herbicides have raised and showed an increase trend from the 1980 decade. Statistics of the United Nations Food and Agriculture Organization carried out from the early XXI century reveal that herbicide consumption, and especially s-triazines, is high both in the European Union and in the US. According to these statistics, US, France and Germany are the main countries that export s-triazine herbicides.
According to the Environmental Protection Agency (EPA), the global market for pesticides during years 2000 and 2001 was more than 2.3 million tons. Only in the US 0.7 million tons were consumed during years 2000 and 2001, which represents about 40% of the herbicide world market and more than 30% of the pesticide world market. In the US, atrazine is the most employed herbicide for weed control in corn, citric, vine and fruit tree plantations, whereas simazine is mainly used in corn crops.
Due to the extensive use of s-triazines for weed control, simazine and atrazine presence has been detected in surface and ground water in several countries such as the US, Switzerland, South Africa and Germany. In Chile, according to the Servicio Agrícola Ganadero de Chile SAG) (Declaración de ventas de plaguicidas de uso agrícola, Ministerio de Agricultura de Chile, 2006), the pesticide consumption during 2004 was about 23,000 tons. Important agricultural plantations (avocado, citruses and grape) in Chile are treated with pre-emergent herbicides such as atrazine or simazine. The use of these herbicides reached 350 tons, representing about 10% of the herbicide used in the country. In the last time, the massive use of agrochemicals has motivated the study of environments that are potentially affected by these compounds. Recent studies have detected s-triazine compounds in agricultural watershed at south-central Chile.
Therefore environmental contamination by s-triazines is a worldwide problem, which requires urgent attention.
One interesting alternative to mitigate s-triazines contamination is bioremediation, which employs microorganisms able to degrade these pollutants or transform them into innocuous compounds for the environment and human health. Some s-triazine-degrading consortia or pure bacterial strains have been isolated, such as Pseudaminobacter, Nocardioides sp. and Agrobacterium. Pseudomonas sp. ADP strain (Mandelbaum and Wacket, 1996) is able to mineralize s-triazines due to specific catabolic pathways well described in the state of the art. Probably, this is the best-known and one of the most characterized strains of the atrazine-degrading microorganisms.
Nevertheless, efficient microorganisms able to degrade s-triazines in the environment are still required.
This technical problem has been solved in the present invention, by the selection of a native bacterium able to degrade s-triazines, i.e. Pseudomonas sp. strain MHP41. In addition, the present invention describes a product that contains strain MHP41 and a method for the bioremediation of s-triazine-polluted environments that uses said product.